Disaggregation process for dynamic multidimensional heat flux in building simulation

Heat transfer across envelopes (façade, roof, glazed areas) represents a big share of the energy flow within the heat balance of buildings. This paper focuses on areas of the envelope where multi-dimensional heat transfer occurs. These areas are commonly defined as thermal bridges, due to a localized reduction of thermal resistance of constructions in these places. This paper reviews common standardized methods to assess heat transfer in buildings, under various modelling assumptions: one-dimensional, multi-dimensional, steady state and dynamic. Within presently developed modelling and assessment methods, a need for improvement has been identified over existing methods for the thermal assessment of multi-dimensional heat transfer under dynamic conditions. A phasorial approach to differential heat transfer in thermal bridges has been developed, which serves as the dynamic extension of steady-state thermal bridge coefficients. This formulation is applied to the junction of a masonry wall with a concrete slab

Proposed Measures to Protect Temporary Roofs from Unwanted Heat Gains

This study focuses on the uncompleted multi-storey residential buildings located in hot climates. This construction pattern is common in the case of incremental housing, where additional floors are added to the building as housing needs grow. Top roofs in these buildings are usually left without thermal insulation until the rest of upper floors are erected. This causes higher thermal discomfort in the top flats compared to the lower ones. Thus, the aim of this study is to investigate thermal effect of some proposed temporary measures that are intended to protect these roofs from unwanted heat gains until the rest of storeys are constructed. This has been carried out using thermal modelling to find out the effect of these measures on the amount of heat transfer through the roof in both summer and winter times. The analysis showed that it is possible to achieve competent thermal protection of the top roof compared to the layered thermal insulation using simple, cost-effective, and reversible measures. Among the examined measures, covering the roof with white foldable sheets and the use of pergolas have been found to be the most effective measures. In both cases, a reduction of 38% in conductive heat transfer through the top roof in summer was observed compared to the unprotected modelling case

Modelling Natural Ventilation in Double Skin Facade

The assessment of the energy performance of buildings with Double Skin Facades (DSF) requires proper dynamic simulation tools, based on models capable of predicting heat and mass transfer in the DSF under variable boundary conditions, at the price of a reasonable computational effort. Many DSF simplified models have been developed and implemented in building simulation tools, but the validation of these tools is still an open issue, especially for the prediction of the mass flow rate in naturally ventilated DSF. The CFD modelling activity presented in this work aims at investigating the reliability of the assumptions and hypotheses employed in the simplified model, which was specifically developed for the dynamic simulation of heat transfer in buildings. Both the CFD and simplified models have been tested and evaluated on an experimental case study, using the database provided by a research program developed under IEA ECBCS Annex 43/SHC Task 34, reporting the results of a measurement campaign conducted on an a transparent naturally ventilated DSF tested in Denmark, in an experimental facility called "the Cube"

Heat transfer and optimum thermal resistance of bulk insulation for naturally ventilated building in tropical climates / Freda Morris

A study was carried out to evaluate the benefits of thermal insulation for naturally ventilated building in a tropical climate of Malaysia. The study was divided into two phases, the field study and simulation study. The field study was conducted in two identical test buildings with dimensions of 4m x 4m x 3m located in Universiti Teknologi MARA Shah Alam. Both test buildings have identical design. The non-insulated was named as Test Building A (TBA) while the insulated was named as Test Building B (TBB) where the insulation was installed consecutively underneath the roof and above the ceiling. This study presents the findings on thermal impact on the non-insulated TBA and insulated TBB. These were appraised by the respective attic and indoor temperature. The comparison of benefit of thermal insulation shows insulation underneath roof was better because the thermal impact for attic and indoor both test buildings indicate an advantage during daytime but ceiling insulation imposed penalty at daytime attic space. The simulation study was run to validate the software as a realistic representation of the real system. Since the respective percentage difference between field study and simulation study 4.2% and 6.1 %, the both data can be compared and new design of modelling will be able to predict other simulation data. This simulation study presents the findings on heat transfer and optimum of insulation thermal resistance to minimize the heat gain through building. The installation of thermal insulation at roof and wall consecutively has reduced the convective and radiative heat transfer but ceiling insulation has decreased the conductive, convective, and radiative heat transfer. The predominant heat transfer proportion through the envelope was via radiation. The determining of optimum thermal resistance of thermal insulation for several models was carried out to minimize heat gain through building. It was concluded that the optimum thermal resistance for thermal insulation installed at roof, ceiling, and wall was model R2 with respective thermal insulation 2.94 m².KW־¹ , 2.86 m².KW־¹ and 2.86 m².KW־¹

uDALES: large-eddy-simulation software for urban flow, dispersion, and microclimate modelling

With continuing urbanization, challenges associated with the urban environment such as air quality, heat islands, pedestrian thermal comfort, and wind loads on tall buildings, are increasingly relevant. Our ability to realistically capture processes such as the transport of heat, moisture, momentum and pollutants, and those of radiative transfer in urban environments is key to understanding and facing these challenges (Oke et al., 2017). The turbulent nature of the urban flow field and the inherent heterogeneity and wide range of scales associated with the urban environment result in a complex modelling problem. Large-eddy simulation (LES) is an approach to turbulence modelling used in computational fluid dynamics to simulate turbulent flows over a wide range of spatial and temporal scales. LES is one of the most promising tools to model the interactions typical of urban areas due to its ability to resolve the urban flow field at resolutions of O(1 m, 0.1 s), over spatial domains of O(100 m), and time periods of O(10 h). Although there are many scalable LES models for atmospheric flows, to our knowledge, only few are capable of explicitly representing buildings and of modelling the full range of urban processes (e.g. PALM-4U Resler et al. (2017); Maronga et al. (2020); or OpenFoam Weller et al. (1998))

Design and Modelling of a Novel Combustion Heat Exchanger for Household Heating

The present study is focused on the design and modelling of a novel Combustion Heat Exchanger (CHE), used for heating and hot water supplies in residential buildings. System design includes a combination of an efficient porous burner and heat exchangers. Combined with an Organic Rankine Cycle (ORC) and a Heat Pump (HP), it is meant to deliver higher energy efficiency as well as reduced greenhouse gas emissions. A numerical model has been developed in STAR-CCM+ to evaluate the design. Furthermore, system level heat transfer calculations were acquired to assist with the design process. A step by step approach was undertaken to investigate physical and chemical phenomena in the system. System dimensions, exchanger location and geometry, air/fuel ratio, porous media models, radiation and combustion were investigated along with different exchanger geometries. A novel spiral heat exchanger was introduced in addition to the common coil designs to exhibit both convection and radiation heat transfers. The results indicated that the exhibition of spiral heat exchanger would result in significantly enhanced heat transfer. Overall heat transfer coefficients of 4-5 times higher in comparison to coils could be expected for spiral exchangers. It was shown that radiation heat transfer accounts for a prominent share in the total heat transfer. Furthermore, the CHE could operate at a wide range of lean air/fuel ratios, enabling further decrease in greenhouse gas emissions. As the last part of the study, further investigations on the regular coil exchangers indicated that these exchangers could still be used with the design, but heat transfer enhancement is required to reduce the dimensions. Such enhancements were tested through shell geometry designs with improved results. Overall, the system shows a promising solution for further reduction of CO2 emissions while improving thermal efficiency

Investigating different modeling techniques for quantifying heat transfer through building envelopes

There is interest concerning the energy performance of buildings in the United States. Buildings, whether residential, commercial or institutional, generally underperform in terms of energy efficiency when compared to buildings that are constructed following sustainably and energy efficiency standards. A substantial percentage of energy loss in these buildings is associated with the thermal efficiency of its envelope (exterior walls, windows roof, floors and doors). The objective of this study will evaluate the results of three energy modeling techniques developed to investigate the energy transfer through the envelope of existing campus buildings. The techniques employed are solving the heat transfer calculations using spreadsheets, using a stand-alone modeling software (OpenStudio) and using an integrated building energy modeling software (eQuest) employed in Autodesk Revit. The first technique is somewhat different from the other two because it does not require a 3D representation of the building to be generated as the first step in the modeling process. It is the application of a mathematical methodology employing heat transfer algorithms entered into the spreadsheet’s cells to estimate the heat transfer through the building envelope. Data needed for this technique are weather data of the buildings location, surface area of the building envelope, and the overall heat transfer coefficient (U-value) of each component of the building envelope. The OpenStudio technique involves a 3D representation of the building. The building is drawn on a 3D modeling computer program called SketchupPro, which communicates directly to the OpenStudio energy modelling interface. The building operations as well as the building characteristics, such as the composition and type of the elements that made up the building envelop, the thermal zone, occupancy schedule and the space type was inputted in the OpenStudio engine. The OpenStudio engine runs the simulation and generates a detail result about the energy usage and energy transfer in the building. The third method that employs AutoCAD Revit software is a standalone technique that does not require an external software for sketching the building model. Revit the ability to draw the model as well as perform the energy analysis at the same time with the aid of inbuilt eQuest modeling engine. The model in Revit is generated with the right building envelope characteristics as the existing building and the weather file. The process is somewhat similar to the OpenStudio technique; the main difference is the level of detail and limitation provided by both the energy modeling engine (eQuest and EnergyPlus). At the end of the simulation, the building energy modeling using Autodesk Revit presents a detailed result of the energy usage and energy flow in the building. The underlying reason of the comparison of three techniques is to understand the simplest, most efficient, accurate method to quantify heat transfer through the building envelope. By the end of this study, the most efficient technique for investigating the building envelope will be expected to be the EnergyPlus technique because of the usage simplicity, ability to take in a lot of details required for simulation and the periodical software updates

Overview of methods to analyse dynamic data

This book gives an overview of existing data analysis methods to analyse the dynamic data obtained from full scale testing, with their advantages and drawbacks. The overview of full scale testing and dynamic data analysis is limited to energy performance characterization of either building components or whole buildings. The methods range from averaging and regression methods to dynamic approaches based on system identification techniques. These methods are discussed in relation to their application in following in situ measurements: -measurement of thermal transmittance of building components based on heat flux meters; -measurement of thermal and solar transmittance of building components tested in outdoor calorimetric test cells; -measurement of heat transfer coefficient and solar aperture of whole buildings based on co-heating or transient heating tests; -characterisation of the energy performance of whole buildings based on energy use monitoring

Improved simulation of phase change processes in applications where conduction is the dominant heat transfer mode

This is the post-print of the Article. The official published version can be accessed from the link below - Copyright @ 2012 ElsevierThis paper reports on the development, experimental validation and application of a semi-empirical model for the simulation of the phase change process in phase change materials (PCM). PCMs are now increasingly being used in various building materials such as plasterboard, concrete or panels to improve thermal control in buildings and accurate modelling of their behaviour is important to effectively capture the effects of storage on indoor thermal conditions. Unlike many commercial simulation packages that assume very similar melting and freezing behaviour for the PCM and no hysteresis, the methodology employed treats the melting and freezing processes separately and this allows the inclusion of the effect of hysteresis in the modelling. As demonstrated by the results in this paper, this approach provides a more accurate prediction of the temperature and heat flow in the material, which is of particular importance in providing accurate representation of indoor thermal conditions during thermal cycling. The difference in the prediction accuracy of the two methods is a function of the properties of the PCM. The smaller the hysteresis of the PCM, the lower will be the prediction error of the conventional approach, and solution time will become the determining factor in selecting the simulation approach in practical applications.This work is funded from the Engineering and Physical Sciences Research Council (EPSRC) of the UK, Grant No: EP/H004181/1